Teaching my classmates

Today I was helping helped my classmates with the their tests. I have class in the morning, and but I went to school in the afternoon again (or you could say "but I went back to school in the afternoon...), because I taught them to teach them (you could also say "tutor" if you were just helping them to better understand what they had already learned from the teacher). They wanted help with physics and chemistry. I would could teach math too, but they wanted help with physics and the time was small short. I hope I've been helpful to them that I helped them.
I discovered that I like to teach, but I don't think I couldn't would be an excellent (I would probably just say "good" here) teacher. I just like to teach because I love to help the people. By the way, I want to be a doctor.

Very good!  I could only find very minor grammar changes.  "Time is short" is an English phrase you'll hear occasionally, but I'm sure you can use it in the past tense too with "Time was short".  My favorite real life example is a quote from the movie Labyrinth.  "It's further than you think... and time is short. You have thirteen hours in which to solve the Labyrinth, before your baby brother becomes one of us... forever. Such a pity..."